Secret communication system



Patented Oct. 29, 1946 UNITED TES ATENT OFFIC Stanley D. Eilenberger, Kenosha,

Wis., assignor,

by mesne assignments, of sixty-five percent to S. E. Steen, half percent to L.

Ohio, and seventeen 16 Claims.

This invention relates to a method of secret communication, principally for military use, where a false or masking signal is provided wherein the false signal in itself is a complete, intelligible signal which may be designed to mislead unauthorized persons, which method has considerable value over so-called masking signals consisting of noise modulation or other spurious frequencies, which in themselves provide evidence that communication of a secret nature is being carried out.

The purposes of this invention are:

To provide a method of secret communication in which the communication shall be disguised in such a manner that reception shall be difiicult for any except the authorized parties.

To provide a method of secret communication employing a true signal lying in the ultra-sonic frequency range and a false intelligible signal lying in the sonic frequency range.

To provide a method of secret communication employing two signals, one of which is the true signal, the other being designated as a false signal, the true signal being so constituted as to be receivable only with special receiving equipment and the false signal being receivable on conventional types of equipment.

To provide a means of secret communication in which two diiferent signals are transmitted simultaneously without any increase in carrier or modulation power beyond that which would be required to radiate only one signal.

This invention will be best understood from a consideration of the following detailed description, in view of the accompanying drawing forming a part of the specification; nevertheless, it is to be understood that the invention is not confined to the disclosure, being susceptible of such changes and modifications as define no material departure from the salient features of the invention as expressed in the appended claims.

In the drawing:

Figure 1 represents one form of a transmitter for use with this system.

Figure 2 represents a receiver for use with the transmitter of Fig. 1.

Figure 3 shows detail of unit X in the transmitter of Figure 1.

For additional detail regarding this system, reference should also be made to my co-pending application, Serial No. 422,915, filed December 13, 1941, entitled Signalling by sub-modulation. All letter designations in the drawing represent units identical with like lettered units in the copending application referred to above, unless Kenosha, Wis., seventeen and one- G. Voorhees, Macedonia,

and one-half percent to Howard W. Taft, Balboa Heights, 0. Z.

Application December 17, 1941, Serial No. 423,384

otherwise shown in this specification, and reference should be made to this co-pending application for this additional detail.

Referring more particularly to Fig. 1, Al represents the source of the true signal which may well be a microphone, or tone modulated telegraph signals, and Bl an amplifier for this true signal input. A--2 represents a source of intelligible false signal and B-2 an amplifier for this false signal. The output of amplifier 13-! is connected to the input of reactance modulator unit C, which is so designed as to frequency modulate controlled oscillator D throughout a range of 20 C. P. S. above and 20 C. P. S. below a mean frequency of kc. The output of controlled oscillator D beats with the output of fixed oscillator E, which is adjusted to a fixed frequency of kc. This provides a beat-note having a mean frequency of 25 kc, and this beat note is frequency modulated between the limits of 25 kc20 C. P. S., by the true signal originating in microphone A-l. If desired, this 25 kc. subcarrier might be obtained directly, without the use of beat frequency oscillators, by adjusting oscillator D to oscillate at 25 kc. In either case, as shown in detail in my above quoted copending application, the resultant carrier will be modulated at an index less than unity, whereby the modulation index is defined as the ratio of frequency deviation from the mean carrier frequency over the signal frequency.

Unit BP is a band-pass filter designed to pass this frequency modulated carrier, together with its associated sidebands. The output of filter BP is connected to input terminal I of switching unit X, while the output of amplifier unit B2 is connected to input terminal 3 of switching uni-t X.

Switching unit X is a two pole switch, with two input terminals I and 3 and two output terminals 2 and 4; this switching unit is arranged that contact must always be between terminals I and 2, or between terminals 3 and 4. Figure 1 shows this switch with contacts closed between input terminal I and output terminal 2, the dotted line from input terminal 3 to output terminal l representing the alternate position of the switch. With switch X in position l2 the output of band-pass filter BP is connected-to the input of modulation amplifier H.

This switch is designed to change positions at a very rapid rate, such as 50,000 times per second. This could be done mechanically by mounting the switching blade on a high speed motor shaft at the center terminal 5 and while such switches are practical, and have been used, particularly in commutator form, a mechanical switch moving at such a high speed is not completely satisfactory. For this reason, an electronic switch is recommended-for this switching unit which accomplishes the same results without moving parts. One simple form of such an electronic switch is illustrated by Fig. 3, and will be described in detail later in this specification.

The balance of the system is arranged for conventional amplitude modulation at any desired carrier frequency, where unit Mist-he modulator, L the power supply, I the carrier-frequency amplifier, E2 the carrier-frequency, oscillator and K the antenna. All of these units are conventional except that modulation amplifier H and modulator unit M should be arrangedto carry out. amplitude modulation at an ultra-sonic frequency, since switching unit X is being moved from the contact position l-2 to the contact position 3-4 atthe rate, of approximately 50,090 timesper second; With this switch in the position 3 i the output of false signal amplifier B-2 is connected directly to the input of modulation amplifier H, andis consequently radiated as a conventional amplitude-modulated signal, which may be received on any conventional type of receiver.

Summarizing the operation of the transmitter of vFig. l, the true signal is radiated as amplitude modulation of the carrier by a frequency-modulated subcarrier havinga mean frequency of 25 kc; and-a-total frequency swing of 40 C. P. S.; the false signal is rad ated-as conventional amplitude modulation. Only one of these signals is present inthe modulator and carrier units at one time, and therefore the modulator and carrier units neednot operate at increased power, even though two separate signals are being radiated.

Referring to Fig-.- 2, there isshown one form.

of 'a receiver for use in recovering the true signal radiated by the transmitter of Fig. 1. In Fig. 2, K represents the receiving antenna, N the radio frequency amplifier, O the convertor, P the intermediate frequency amplifier, and Q the amplitude demodulator; all of these units are conventional, except, in the example given, the acceptance band of units N, O, P, Q must be at. least wide enough topass the received carrier and its associated sidebands, including the switching frequency, which, in the example given,is 50110.; the carrier and its associated sidebands alsooccupy 50 kc, if both sidebandsare transmitted. If desired, either the upper or lower sideband may be removed by any of the commonly used methods. The original frequency modulated subcarrier, having a mean frequency of 25- kc., is present at the output of demodulator Q, also the original signal frequencies impressed as the false signal at microphone A-2 at the transmitter of Fig. 1; this combined output of demodulator Q is passed through high pass filter S-l which has, for example, a low frequency cut-off at 5000 cycles, which eliminates the so-called false signal, and passes the 25 kc. subcarrier on to amplifier-mixer unit F. Here it beats with a constant frequency from fixed oscillator E, which has a frequency of 24.9 kc., so that the resultant beat note at the output of, mixer unit F is a frequency modulated audio frequency subcarrier having a mean frequency of 100 cycles. This frequency modulated 100 cycle subcarrier is impressed on filter unit G, which isa parallel resonant filter with a resonant frequency of 108 cycles, identical in function and purpose to filter G described in detail in my above quoted co-pending application.

Unit R is a. frequency discriminator designed with a mean resonant frequency of cycles and further designed to provide eflicient recovery of the modulating signal on the 40 C. P. S. frequency swing. Unit S2 represents a high pass filter with a low frequency cut-01f at cycles, designed to remove residual subcarrier frequencies. Unit T represents the audio frequency amplifier, and U the reproducer. The reception of such a modulated carrier wave, which has been amplitude modulated by a frequency modulated subcarrier, is fullyv describedin my co-pending application referred to above.

A full study of, this co-pending application Will reveal'that numerous additional methods of disguising the true signal are possible and it should be further. understood that any existing type of disguising a signal, such as frequency inversion, might be used in lieu of the system shown here. If such alternate method were employed the apparatus used'to disguise the signal would baconnected between the output terminal of amplifier B1.- and' the input to switch terminal I. A requirement of such alternate method is that the switching operation and the converted true signal, as supplied to the input of modulation amplifier H, be at on ultra-sonic frequency.

Figure 3 shows adetailed arrangement of a simplified electronic switch-for use as switching unitzX'inthe transmitter of Figure 1. Terminals No, l and 3 represent the input-terminals and terminals No. 2 and' trepresent theoutput terminals, these terminalsbeing numbered identically' with the arrangement shown in Fig. 1. Thevacuum tubes-9 and- Hl might be commercial type 6L7 or any other triple grid type arranged as an amplifier; 9 isthea-mplifier for input terminal! and if is the amplifier for input terminal 3. fliese tubes-are arranged to alternately amplify andblockatthe frequency of the pulses supplied from square-wave generators SW-i and SW--2, the frequency and synchronization of which arecontrolled by unit PG, which is a pulse generatoror oscillator, operatingat-the desired switching frequency, which'in this case is- 50,000 times persecond. Blocking pulses are supplied to auxiliary control-grids l3 and l' tso that the signal inputs received at prirn-a'ry' Contr01- grids H and i2 are alternately amplified, and the signal voltages developed across resistors l'lapplied through coupling condensers E8 to output terminals z and l. Condenser i9 is arrangedto by-pass any inter-coupling between stages. Terminal 2? represents the common ground connection-and terminal 21 a source ofhigh voltage of approximately 250 volts. Cathode, primary control grid and screen grid circuits are conventionallyarranged. Electronic switchingdevices similar to that illustrated by Figure 3 are in common-use and commercial units may be obtainedoperating onthis principle.

While the foregoing description servesto illustratethe manner in which the objects'of my invention may be carried-out, nevertheless, it is understood that I do not desire myinvention to be limited thereby, but only bythe-scopeofthe following claims.

I claim:

1.- The method of secret communication, which comprises the steps of p-roducing an ultra-sonic tellignce, and alternately amplitude modulating a'high frequency carrier wave bythe frequency modulated ultra-sonic carrier wave and by the false signal containing any arbitrary intelligence at an'ultra-sonic rate of alternation;

'2. The method of secret communication, comprising the steps of producing a first high frequency carrier Wave, frequency modulating said first'high frequency carrier wave by a true signal containing the intelligence to be transmitted to an" authorized receiver at a modulation index smaller than unity, beating said frequency modulated high frequency carrier with a constant frequency wave to produce a frequency modulated ultra-sonic carrier, producing a false signal containing any arbitrary intelligence, alternately amplitude modulating a second high frequency carrier by both the frequency modulated ultrasonic carrier and the false signal at an ultrasonic rate of alternation.

3. The method of receiving a true signal containing the intelligence transmitted to an authorized receiver by the method described in claim 1, comprising the steps of demodulating the amplitude modulated high-frequency carrier wave, filtering the demodulated wave to attenuate all the frequency components contained in the false signal and to obtain the modulated ultra-sonic wave, demodulating the ultra-sonic Wave to recover the true signal containing the intelligence intended for reception.

4. Themethod of secret communication, Which comprises producing a true signal and an intelligible false signal of sonic frequency, converting the true signal to an ultra-sonic signal frequencymodulated at an index less than unity, combining the sonic signal and the ultra-sonic signal, transmitting thecombined signals to a receiving point, eliminating the sonic signal, and reconverting the ultra-sonic signal into the true signal.

5. The method of secret communication, which comprises producing an intelligible true signal and an intelligible false signal, converting the true signal into an ultra-sonic signal frequencymodulated at an index less than unity, alternately transmitting the ultra-sonic signal and the false signal at an ultra-sonic rate of alternation to a receiving point, eliminating the false signal and reconverting the ultra-sonic signal to the true signal.

6. A system of secret communication, comprising a source of intelligible true signals, a source of intelligible false signals, means for converting the true signals into ultra-sonic unintelligible signals frequency-modulated at an index less than unity, means for alternately transmitting the intelligible false signals and the ultra-sonic unintelligible signals to a receiving point, means for eliminating the false signals, and means for reconverting the ultra-sonic unintelligible signals to true signals.

7. The method of receiving a, signal containing the intelligence transmitted to an authorized receiver by the method described in claim 1, comprising the steps of demodulating the amplitude modulated high frequency carrier, filtering the demodulated wave to attenuate all the frequency components contained in the false signal and to obtain the frequency modulated ultra-sonic carrier, and demodulating said frequency modulated ultra-sonic carrier to recover the true signal.

8. The method of receiving a signal containing the intelligence transmitted to an authorized receiver by the method described in claim 1, comprising the steps of demodulating the amplitude modulated high frequency carrier wave,. filteringv the demodulated wave through a high pass filter to attenuate all frequency components contained in. the false :signal and to. obtain the frequency modulated ultra-sonic carrier, beating the recov-' ered ultra-sonic carrier with a constant frequen-. cy Wave to produce an "audio frequency frequencyv modulated subcarrier, and demodulating theatre-'- quency modulated subcarrier to recover the true signal; I

9. In, a transmitting circuit" a source of-true signal; .a source of false signal, a high frequency oscillator frequency modulated at an index. less than unity by the true signal, a high speed change-over switching circuit, the input sides of which being connected to receive both true and false signals, and a high frequency oscillator amplitude modulated by the output of the high speed switchingcircuit.

10. In a transmitting circuit a source of true signal, a source of false signal, a source of ultrasonic oscillations and means for frequency modulating said ultra-sonic oscillation at a modulation index less than unity by the true signal, a source of high frequency oscillations and means for alternately causing the false signal and the frequency modulated ultra-sonic oscillations to modulate the high frequency oscillations at an ultra-sonic rate of alternation,

11. The method of receiving a signal containing the intelligence transmitted to an authorized receiver by the method described in claim 2, comprising the steps of demodulating the amplitude modulated high frequency carrier, filtering the demodulated Wave to attenuate all the frequency components contained in the false signal and to obtain the frequency modulated ultra-sonic carrier, and demodulating said frequency modulated ultra-sonic carrier to recover the true signal.

12. The method of receiving a signal containing the intelligence transmitted to an authorized receiver by the method described in claim 2, comprising the steps of demodulating the amplitude modulated high frequency carrier Wave, filtering the demodulated Wave through a high pass filter to attenuate all frequency components contained in the false signal and to obtain the frequency modulated ultra-sonic carrier, beating the recovered ultra-sonic carrier with a constant frequency Wave to produce an audio frequency frequency modulated subcarrier, and demodulating the frequency modulated subcarrier to recover the true signal.

13. The method of secret communication which comprises producing a true signal and an intelligible false signal of sonic frequency, converting the true signal toan ultra-sonic signal frequencymodulated at an index less than unity, combining the sonic signal and the ultra-sonic signal, modulating a carrier Wave by the combined signals,

transmitting the carrier Wave to a receiving point, then demodulating the carrier Wave to recover the combined signals, eliminating the sonic signal, and reconverting the ultra-sonic signal into the true signal.

14. The method of secret communication which comprises producing an intelligible true signal and an intelligible false signal, converting the true signal into an ultra-sonic signal frequencymodula'ted at an index less than unity, alternately modulating a carrier wave by the ultra-.

sonic signal and the false signal at an ultra-sonic rate, transmitting the carrier Wave to recover the ultra-sonic signal and the false signal, eliminating the false signal and reconverting the ultrasonic signal to the true signal.

15. A secret communication system comprising a source of true signals, a source of intelligible false signals, means for converting the true signal 'tora'n ultra-sonic signal frequency-modulated atan index less than unity, switching means for alternately introducing the false signal and the ultra-sonic signal into a channel for transmission to a receiving point, means at the receiving point for eliminating the false signal, and means for reconverting the ultra-sonic signal to the tru signal.

16. .A secret communication system comprising a sourceof true signals, a source of intelligible false signals, means for converting the true signal to an ultra-sonic signal frequency-modulated at an index less than unity, means for producing a carrier wave, switching means for alternately causing the false signal and the ultra-sonic signal to modulate the carrier wave for transmission to a receiving point, means at the receiving point for demodulating the carrier Wave to recover the false signal and the ultra-sonic signal, means for eliminating the false signal, and means for re converting the ultra-sonic signal to the true signal.

STANLEY D. EILENBERGER. 

